Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuit. One important application is the sputtering of barriers and other liner layers on the sides of high aspect-ratio via holes. One method of achieving deep penetration into the via hole is to produce a high ionization fraction of sputtered atoms, that is, a high fraction of ions in the sputter flux, and to RF bias the wafer to attract the ions deep within the hole. Similar techniques are used to sputter etch somewhat resistive barrier material at the bottom of the via with the accelerated metal or argon working gas ions or somewhat equivalently to balance the sputter etching and sputter deposition at the bottom of the via while primarily sputter depositing on the via sides.
A magnetron sputter chamber 10 illustrated schematically in cross section in FIG. 1 can effectively sputter thin films of the refractory metals Ta, Ru, or RuTa or their respective nitrides into holes having high aspect ratios and can further act to plasma clean the substrate and selectively etch portions of the deposited refractory-based films. Sputtering of copper seed layers can similarly be effected. The sputter chamber 10 includes a vacuum chamber 12 arranged generally symmetrically about a central axis 14. A vacuum pump system 16 pumps the vacuum chamber 12 to a very low base pressure in the range of 10−6 Torr. However, an argon gas source 18 connected to the chamber through a mass flow controller 20 supplies argon as a sputter working gas. The argon pressure inside the vacuum chamber 12 is typically held in the low milliTorr to high sub-milliTorr range at least during ignition of the plasma. A nitrogen gas source 22 supplies nitrogen gas into the vacuum chamber 12 through another mass flow controller 24 when a refractory nitride is being deposited. An electrically grounded shield 26 protects the chamber walls from sputter deposition and also acts as the anode in generating the plasma.
A pedestal 30 arranged about the central axis 14 holds a wafer 32 or other substrate to be sputter coated. An unillustrated clamp ring or electrostatic chuck may be used to hold the wafer 32 to the pedestal 30. An RF power supply 34 supplies RF power through a capacitive coupling circuit 36 to the pedestal 30, which is conductive and act as an electrode. In the presence of a plasma, the RF biased pedestal 30 develops a negative DC bias, which is effective at attracting and accelerating positive ions in the plasma whether refractory metal or argon ions. The grounded shield 26 is cup-shaped to also protect the sides of the pedestal 30 from sputter deposition. A target 38 is arranged in opposition to the pedestal 30 and is vacuum sealed to the vacuum chamber 12 through an isolator 40. At least the front surface of the target 38 is composed of a metallic material to be deposited on the wafer 32, in this case, ruthenium, tantalum, or a ruthenium tantalum alloy. For a copper seed layer, the target 38 is composed principally of copper.
A DC power supply 42 electrically biases the target 38 to a negative voltage with respect to the grounded shield 26 to cause the argon to discharge into a plasma such that the positively charged argon ions are attracted to the negatively biased target 38 and sputter tantalum or ruthenium atoms from it, some of which fall upon the wafer 32 and deposit a layer of the refractory target material on it. In reactive sputtering, reactive nitrogen gas is additionally admitted from the nitrogen gas source 18 into the chamber 12 to react with the tantalum or ruthenium being sputtered to cause the deposition of a refractory metal nitride layer on the wafer 32.
Although some advanced sputter chambers include an RF inductive coil to increase the plasma density or to generate an argon plasma for sputtering, none is used here in the described embodiment of the chamber.
The target sputtering rate and sputter ionization fraction of the sputtered atoms can be greatly increased by placing a magnetron 50 is back of the target 38. The magnetron 50 preferably is small, strong, and unbalanced. The smallness and strength increase the ionization fraction and the imbalance projects a magnet field into the processing region for at least two effects of guiding sputtered ions to the wafer and reducing plasma loss to the walls. Such a magnetron includes an inner pole 52 of one magnetic polarity along the central axis and an outer pole 54 which surrounds the inner pole 52 of the opposite magnetic polarity and separated from the outer pole 54 by an annular gap 56. The poles 52, 54 are supported at their backs by a magnetic yoke 58, which magnetically couples them.
The magnetic field extending between the poles 52, 54 in front of the target 38 creates a high-density plasma region 60 adjacent the front face of the target 38, which greatly increases the sputtering rate. The high-density plasma region 60 generally follows the annular gap 56 and hence forms as a closed plasma track having no end loss. Conventionally, the high-density plasma region 60 is relatively shallow and confined to near the target 38. Some aspects of the invention concern the size and shape of the high-density plasma region 60. The magnetron 50 is unbalanced in the sense that the total magnetic intensity of the outer pole 54, that is, the magnetic flux integrated over its area, is substantially greater than that of the inner pole, for example, by a factor of two or more. The unbalanced magnetic field projects from the target 38 toward the wafer 32 to extend the plasma and to guide sputtered ions to the wafer 32 and reduce plasma diffusion to the sides.
Early versions of the magnetron 50 were typically formed in a triangular shape, that is, asymmetrical about the central axis 14. A motor 62 drives a rotary shaft 64 extending along the central axis 14 and fixed to a rotation arm 66 supporting the yoke 58 and the magnetic poles 52, 54 to rotate the magnetron 50 about the central axis 14 and produce an azimuthally uniform time-averaged magnetic field. However, later versions of the magnetron 50 have a substantially circular shape and a small size relative to the target and they are often placed to overlie the outer portions of the wafer 32. Magnetron systems are known in which the radial position of the magnetron can be varied between different phases of the sputtering process and chamber cleaning as described by Gung et al. in U.S. patent application Ser. No. 10/949,735, filed Sep. 29, 2005 and published as U.S. Patent Application Publication 2005/0211548, incorporated herein by reference in its entirety.
For many liner applications, the magnetron 50 is small, as illustrated in the schematic view of FIG. 2, and located over the outer portion of the wafer 32. The unbalanced magnetic field 70 projecting from the magnetron 50 towards the wafer 32 tends to guide the ions towards the center 14 of the wafer. As a result, a typical ion flux profile 72 illustrated in FIG. 3 tends to be distinctly heavier at the center. Whether for sputter deposition or sputter etching, the flux profile results in non-uniform deposition or etching. Furthermore, the guidance field 70 of FIG. 2 and hence the trajectories of the ion even after acceleration across the plasma sheath towards the biased wafer 32, tend to be more strongly angled near the wafer center 14 than at the wafer edge. Even if auxiliary magnetic coils or sidewall magnets or magnets under the wafer can even out the ion flux profile 72, the angular non-uniformity introduced by the guidance field 70 results in sidewall asymmetry and via bottom shielding.